Coastal mapping - a simple guide: Liverpool

Updated Tuesday 6th November 2007

A simple guide to changes in Liverpool Bay. For a richer experience, explore these changes with our coastal mapping interactive.

The Port of Liverpool grew rapidly from humble beginnings in 1715 to become one of the largest ports in the world. Today, docks span seven and a half miles of waterfront, and over 30 million tonnes of cargo and 720 000 passengers pass through the port each year.

The success of the port had much to do with maintaining the deep channels in Liverpool Bay which allowed the passage of ever-larger ships into the River Mersey. So it was essential to tame the influence of the sand banks which shifted around the bay.

The five charts presented here outline the changing shape of the sand banks and channels in Liverpool Bay. Together, they illustrate a delicate balance between the forces of nature and the need for increasing human influence.

1862 map

Copyright: National Maritime Museum

The five charts presented here record the movement of sediment into, out of, and around the Outer Estuary of Liverpool Bay between 1862 and 2001. Note that the charts are oriented with north to the left, so Liverpool lies just off the top right-hand corner of each chart (the docks being visible on later charts).

The sediments within the Outer Estuary have been around since the last glaciation 10 000 years ago. As the glaciers retreated and sea level rose, sediment in the Irish Sea migrated eastward under the influence of waves and tides. Today these sediments form large banks of sand (with some gravel); these are shown in yellow on each map.

Each sand bank lies within the inter-tidal area and is therefore exposed during the lowest tide and submerged during the highest. Between each of the banks are channels of deeper water, which have been important shipping thoroughfares since the first dock opened on the River Mersey in 1715.

Embankments fringe much of the shoreline bounding the estuary and this prevents erosion. However, Formby Point is composed of unconsolidated windblown sand dunes susceptible to erosion during storms.

Waves, tides and storms have controlled the build up of sand banks and the cutting of the channels. However, attempts to maintain a deep channel for shipping between Liverpool and the Irish Sea (through dredging and the construction of training walls) have upset this natural balance. This is well illustrated by the following sequence of charts.

1862

Copyright: Open2 team

The first chart, from 1862 represents a period of net sediment accumulation on the inter-tidal sand banks. A number of small temporary channels crossed Great Burbo Bank and the channel between Jordan’s Bank and Taylor’s Bank reached a maximum depth of 10 metres.

Both the Formby Channel in the northeast and the Rock Channel to the south were up to 15 metres deep and provided alternative navigable routes into port for smaller ships. However, with a depth of up to 20 metres, the dominant channel for shipping was the Crosby Channel. The width of this channel was governed by the migration of Taylor’s Bank and Great Burbo Bank.

The concern at the time would have been that as Liverpool was competing for Atlantic trade, the influence of the Crosby Channel might wane to be replaced by a network of smaller channels around Jordan’s Bank and Taylor’s Bank. For this reason, limited dredging of the channel was carried out.

1880

Copyright: Open2 team

By 1880, the Crosby Channel had shifted slightly northwards, cutting into Taylor’s Bank. Formby Channel had straightened somewhat and was still navigable by smaller ships. Great Burbo Bank continued to grow in size and extended further out into Liverpool Bay closely flanking the Crosby Channel at its mouth. Likewise, sand on Taylor’s Bank began to form a spit at its most seaward extremity.

Between 1862 and 1880, dredging of the Crosby Channel and the disposal of spoil on Great Burbo Bank began to influence the shape of the channels and sand banks. However, the natural addition of sand to the inter-tidal area continued, most probably during high spring tides and storms, which brought up sediment from deeper areas further out in the bay.

Formby Point was building out seaward at this time, so sediment from the estuary contributed to the formation of windblown dunes on the Point.

Rivers normally replenish their estuaries with sediment, however, the limited supply of sediment from the River Mersey is not thought to have influenced the sand banks and channels of the Outer Estuary.

1880 Source map

Copyright: National Maritime Museum

1920

Copyright: Open2 team

The construction of a training wall of limestone blocks commenced in 1909. As a result, by 1920, the channel had deepened to more than 25 metres and induced a westward curvature in its shape as it entered the Irish Sea.

Between 1912 and 1946, the inter-tidal area lost around 15% of its total sediment volume. This is illustrated by a decrease in size of most of the sand banks, especially Great Burbo Bank (despite being the site of dredge spoil dumping).

The loss of sediment around this time has been linked to winter gales, which were especially frequent between 1880 and 1905. Individual storms caused significant erosion of the sand banks and redistributed the sediment along the shorelines of the estuary. Mad Wharf was greatly reduced in size and after 1909 storms began to erode Formby Point (a trend which has continued unabated with 0.5 km of shoreline eroded from the Point to the present day).

By 1920, Formby Channel had diminished greatly in importance, due perhaps to the longshore drift of sand from Mad Wharf. Taylor’s Bank was greatly reduced in size, despite becoming a major focus for dredge spoil dumping. (Between 1913 and 1936, 456 million tonnes of dredged material was dumped to the north of Taylor’s Bank.)

To the south, the Rock Channel off Mockbeggar Wharf, which had been navigable to shipping since before 1862 started to shallow. In its place, a new channel opened up to the east, entering the bay just west of the Great Burbo Bank. This ‘New Rock Channel’ maintains a depth of about 15 m to the present day.

1920 Source map

Copyright: National Maritime Museum

1946

Copyright: Open2 team

By 1946, construction of the training wall was nearing completion. With walls now lining both sides of the Crosby Channel, it became effectively isolated from the adjacent sand banks. This helped to maintain depths of 25 metres within the channel.

The training walls and dredging/dumping of spoil heavily influenced the shape of the sand banks and distribution of channels. On ebb tides, currents were concentrated in the Crosby Channel, drawing sand out into the bay, whilst storms and flood tides brought sand up onto the adjacent sand banks on either side of the channel. As a consequence, the annual volume of sand dredged from the channel declined from 25 m tonnes in 1924, to between 5 and 9 m tonnes in the years around 1946.

Formby Channel, which had been in decline up to this point, now infilled at an accelerated rate, helped by the migration of dredged spoil from Taylor’s Bank. The Rock Channel also infilled markedly, due mainly to the southward migration of dredged spoil dumped on Great Burbo Bank.

1946 Source map

Copyright: National Maritime Museum

2001

Copyright: Open2 team

Between 1946 and 2001, the training wall construction was finally completed with a small westward extension out into the bay (1957). In addition, the height of the training wall was raised by 2 metres after sands began to overtop the wall (1962).

By 2001, the sand banks were continuing to adjust to the influence of the training wall. They had become more laterally continuous, yet clearly pressed back against the coast and training wall, with the result that minor channels have become completely infilled. Indeed the only channel to remain open without human intervention was the New Rock Channel.

The general east–west alignment of the sandbanks suggests that the shape of these banks was controlled by flood tides and westerly waves crossing the Irish Sea. As in 1946, the ebb tides concentrate in Crosby Channel, with the flood tides dominating the shape of the adjacent sand banks.

It is estimated that the volume of sediment on the sand banks decreased by a further 6-7% between 1946 and 2001. Whilst longshore drift and the movement of sediment further offshore may have contributed to this loss, it is thought likely that much of the sediment lost from the Outer Estuary sand banks has moved further upstream within the tidal reaches of the River Mersey.

A total sea level rise of 10 cm in the last century is not thought to have contributed significantly to the changing face of the Outer Estuary. However, future increases in sea level, currently estimated at 4 mm per year to 2030 and the intensity of storm surges may cause further loss of sediment from the estuary.

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